This thesis investigates how fluent bilinguals make use of the grammar of their two languages when they construct verb- and pronoun-agreement only in one language (monolingual mode) or in both their languages (bilingual mode). We are particularly interested in the impact of the non-response language in sentence processing on the response language. Bilingual research has provided evidence for language integration in bilingual speech (e.g., Hartsuiker, Pickering, & Veltkamp, 2004) which is also consistent with the phenomenon of code-switching whereby speakers can use elements of each language in producing mixed-language utterances (e.g., Myers-Scotton, 2002). So far, studies at the lexical level have provided support for parallel language activation (e.g., Colomé, 2001), yet the issue of whether activation of either language can be strong enough to influence the workings of the other is still in dispute (e.g., Hermans, Bongaerts, de Bot, & Schreuder, 1998, but see Costa, La Heij, & Navarrete, 2006). In three separate sections of the thesis we employ a sentence-completion paradigm widely used in monolingual agreement literature (Bock & Miller, 1991) to examine language interaction effects in the monolingual and the bilingual modes of speech (Grosjean, 2000). English-Greek and Greek-English fluent bilinguals produced completions to singular or plural subjects when the number of the translation was either the same or different, and when their completion either did or did not switch languages. The first section investigates whether there is influence of the divergent number properties of the nonresponse native language (L1) on verb-agreement in the response second language (L2). The results of Greek-English bilinguals show influence of the underlying number of the L1 on completions in the L2. We interpret this in terms of a markedness account (e.g., Eberhard, 1997) whereby parallel activation and competition between an L2 singular subject noun and its L1 plural translation results in plural verbagreement because the singular form is more vulnerable to the marked plural form. English-Greek bilinguals who perform on the same monolingual mode do not show influence of their L1 when speaking in the L2 (Greek). We attribute this finding to a difference of morphological/inflectional properties between the two languages which renders a language that displays fewer overt markings (English) easier to control when utterances are produced in a language that displays more overt markings (Greek) (e.g., Vigliocco, Butterworth, & Semenza, 1995).